But I can say that there will come a time when Bitcoin will be the most used currency, because as time goes by, fiat money is decreasing. Now people are afraid to carry paper money and go on the streets because there is a high chance of getting robbed, using Bitcoin is the safest because the passenger sitting next to you will never know if you have Bitcoin.
We are moving towards the future and using modern transactions more, it can be Bitcoin or we complete transactions by numbers, so the use of fiat currency is decreasing.
Nothing stops anyone from having Bitcoin and fiat, know this that Bitcoin can't replace fiat no matter our level of adoption.
Paper money isn't only called fiat, the balance in our banks are part of fiat, there are more electronic banks for fast spending and less charges. I don't believe banks and fiats are getting replaced, Bitcoin remains an alternative for fiat but likely to grow wide for investment purpose.
People are going to use Bitcoin for investment more than they will as currency.